Method of recovering magnesium compounds



April l0, 1945. T c. ATcHlsoN METHOD OF RECOVERING MAGNESIUM COMPOUNDS Filed Aug. 15, 1941 A TORNEY Patented Apr. 10,1945

Thomas C. Atchison, Princeton, J.; assiznor to y Johns-Manville Corporation, New corporation of New York York, N. Y., a

Application Angustia, 1941, serial No. 406,5?17

Claims.

l This invention relates to magnesium carbonate compositions of the type suitable for insulating purposes, It -relates particularly to a methodl of wasted when pipe lines, boilers, etc.. are stripped of old magnesia insulating materials. Also, minrecovering magnesium compounds from materials v previously wasted andutilizing the recovered may terials in the process of manufacturing magnesium carbonate compositions in the conventional process heretofore practiced. l

Heat insulating materials containingl basic magnesium carbonate are generally made by forming an admixture of basic magnesium carf bonate and asbestos bres, the libres being present to impart binding characteristics and additional strength to the product.'The mixture of the basic carbonate and fibres is placed in suitable molds shaped in accordance with the desired era1s, either calcined or uncalcined, containing relatively large proportions vof magnesia values,

-suchV as serpentine, chrysotile, etc., but which are diicult to carbonate, maybe utilized as a sourcev of raw material in the operation of my present invention. Through the use of the carbonating procedure, as will be described in more detail below, the magnesia components in these products, some of which were previously considered waste materials, may be readily recovered, vand design of the iinal product,A and thereafter the molded product is subjected to a drying action.

. informing the heat insulatingmaterial the proportions of the bre and the basic magnesium carbonate are so proportioned that the final prodthe resulting recovered magnesium compounds. may be used in preparing new magnesium compounds for various purposes. y

It is, therefore, an object of this invention to recover magnesium compounds formerly lost in waste materials and: sludge which will thus inuct containsabout 85 per cent of basic magne-`- 'slum carbonate. This product is generally reyferred toA infconimerce as 85 per cent :magnesia` In preparing "the basic magnesiumcarbonate used in 85 per cent magnesia, a considerable proportion of the magnesium compounds obtained asy a result of following the conventional procedure ormanufacturi'ng magnesium carbonate mate.

Arialsvare lost in waste water and waste sludges. It has been estimated that magnesium compounds to the extent of -25 per cent of vthe i magnesia values in thev raw materials havebeen 'It is known that an aqueous suspension of caly. l

' cium carbonate and magnesium carbonate, as, for

lost in the wastel waters andsludge which have always inthe pastbeen discarded. Y recover,- ing the magnesia values from the aste-waters .and vsludge a considerable reduction .in cost of the manufacturingl operations will be obtained.

example,'80 per cent calcium carbonate and 20 per cent .magnesium carbonate, could betreated i with a large excess of vstrong carbon dioxide gas to dissolve the magnesium vcarbonate forming aA magnesium bicarbonate solution and leavingthe calcium carbonate undissolved, but no economical method has thus far been developed to apply 'the'above separation to plant operations.

Large quantities of magnesium carbonates have also in the-past been wasted inthe form of trimmings obtained during the sizing and finishing operations' of crude molded basic magnesium car bonat e 'products. These trimmings are usually in the lornffadust and small lumps. Additional .quantitiesof magnesium compounds havebeen crease the plant capacity for the production of magnesium carbonate products by about 20`25 per cent. The plant capacity will be increased to Ithis degree, since the volume of Athe materials passing through thevequipment need not, 'be increased Ain order to make Ythe recovery oi' the magnesium compounds from the waste products,

effective. The recovered products are merely entered into the regular production cycle and the concentration ofthe magnesium 'solutions may be increased without detrimental eifects which were previously encountered prior to the utiliz- -ation of the present invention.

It is also an object of this invention to recover the magnesium compounds formerly lost-in waste materials at a negligibly increased cost in operating expenses. The recovery operation may also be applied to recover magnesia values vfrom trimmings, old magnesia insulation materials 'and difficultly carbonatable minerals. The necessary equipment for carrying on the present process lis not very expensive, and the increased overhead expense as a result of the equipment and operating costs will be very small as compared to the value of the magnesium compounds now recove ered but previously lost.

It is a further' object of. this invention to ma- "terially reducethe amount of steam required for precipitating the magnesium bicarbonate so- .lutions, inasmuch as theconcentration of the magnesium' bicarbonate'solution can be materially increased. Previously, it was undesirable to have too high concentrations of the magnesiumj bicarbonate solutions because of the fact that a substantial proportion of the magnesium compounds precipitated; from the solution and were thereby lost in the waste waters andrsludge.-

However, through the application of the present novel procedure more fully described herein, the resulting' increase in decomposition of the magnesium bicarbonate solutions is no longer of any significance, since the decomposed material is recovered. Thus, since the magnesium bicarbonate solutions may now be used at higher concentrations, -the amount of steam formerly required for boiling magnesimn bicarbonate solutions to precipitate the magnesium carbonate may be reduced.

Additional objects and benefits of the present invention will be apparent to those skilled in the art from the more detailed description of the novel method hereinafter set forth.

Broadly stated, the present invention includes the treatment oi' waste sludges and otherdiflicultly carbonatable materials containing retained or absorbed soluble magnesium carbonate vand insoluble magnesium carbonate with carbon dioxide gas to form water soluble magnesium compounds. The water-soluble magnesium com- `pounds are then separated from the water-insoluble compounds, and the resulting solution containing the Water-soluble magnesium compounds is used in conjunction with fresh magnesium compounds in the regular manufacturing cycle.

'I'he invention will be illustrated by la, more complete description in connection with the attwched drawing, which shows a ow sheet of the y preferred embodiment of the method for recovering magnesium compounds from waste materials. is the conventional method heretofore used in manufacturing basic magnesium carbonate products. The portion of the flow sheet in heavy lines illustrates the procedure followed in my invention in recovering the magnesium compounds from the waste materials.

In the drawing illustrating a conventional method of manufacturing basic magnesium carbonate, dolomite mineral (composed primarily of f magnesium carbonate and calcium carbonate) and coke are fed to a kiln and subjected to relatively high temperatures to convert the magnesium carbonate and the calcium carbonate to magnesium oxide and calcium oxide, respectively. During the calcining action, carbon dioxide gas is evolved as a result of the reduction of the magnesium carbonate and calcium carbonate to the respective oxides. The CO2 gas is utilized in the high concentration carbonator and the low concentration carbonator, as will be described in more detail below in the subsequent steps of theoperation.

After the dolomite ore has been sufficiently calcined to form magnesium oxide and calcium oxide, the materials are subjected to a slaking operation. In this operation water is mixed with the oxide mixtures'and thereafter the slurry of the slaked materials, composed primarily of the magnesium hydroxide and calcium hydroxide, is then pumped to a series of high concentration carbonating tanks where the mixed hydroxides are subjected to the action of carbon dioxide gas. In the high concentration carbonating tanks. the concentration of the magnesia is in excess of 1.5 per cent expressed as basic magnesium carbonate (approximately six grams MgO per. liter, or 0.6 per cent). The carbon dioxide gas is bubbled through the magnesium hydroxide and calcium hydroxide mixture in suflicient quantity to react with the magnesium and calcium compounds to form carbonate compounds. The substantially water-insoluble magnesium hydroxide The broken line portion of the flow sheet r 'IISA is converted into a water-soluble magnesium bicarbonate and the calcium hydroxide is converted into a substantially water-insoluble calcium carbonate. l

The magnesium bicarbonate solution containing the water-insoluble calcium carbonate is thereafter pumped to a settling tank wherein the water-insoluble components are permitted to set tle to the bottom of the tank. A small percentage of the magnesium bicarbonate solution has a tendency to precipitate in the form of normal magnesium carbonate (MgCOaI-IzO) which will settle with the calcium carbonate components in the bottom of the tank.

The substantially clear and sludge-free magnesium bicarbonate solution is then pumped to a precipitator and the solution is subjected to a heating operation. During the heating the magnesium bicarbonate will precipitate from the solution in the form of an insoluble magnesium carbonate which may be in the form of normal magnesium carbonate or the basic magnesium carbonate, and this reaction is accompanied by the evolution of considerable carbon dioxide. The precipitated magnesium carbonate is then placed into a filter or other dewatering device to remove the major portion of water, and the magnesium carbonate slurry is thereafter placed into suitable molds of any desired shape, followed, if necessary, -by the application of pressure. If fibres are added to the material, the fibres `may be added after a substantial proportion of the water has been removed from the slurry, that is, after the slurry has been'passed through the filter.

The lime sludge obtained from the lime sludge thickener contains a substantial proportion of precipitated magnesium carbonate, retained or absorbed magnesium bicarbonate solution, calcium carbonate, and other compounds. The percentage of magnesia values in these sludge products, previously Wasted, is very high, and it is advantageous to recover these magnesio.' values in order to increase the plant capacity over normal operations.

The sludge obtained from the lime sludge thickener is mixed with additional. water until the mixture forms a comparatively thin slurry. It is preferable that the slurry be diluted with water to such a degree that the concentration of the magnesium carbonates is at least below 1.5 per cent expressed as basic magnesium carbonate (approximately six grams of MgO per liter or 0.6 per cent). It has been found that if a, slurry be diluted still further so that the concentration of the magnesium carbonates would be approximately .5 per cent concentration expressed as basic magnesium carbonate (two grams MgO per liter o'r 0.2. per cent), more satisfactory results are obtained. Highly dilute solutions are desirable in order to obtain eiective separation of the magnesium compounds from the insoluble calcium compounds; If the concentration of the magnesium carbonates is too high, a considerable proportion of the magnesium compounds will be lost with the calcium carbonate sludge which is discarded in a subsequent operation of the present process.

Trim materials, waste or discarded basic magnesium carbonate materials, diflicultly carbonate.- ble minerals, or other products containing magnesia values, are added at this point of the operation if such products are to be used in the operation. It is desirable to break or crush the products into a relatively iine condition and suspend the pulverulent materials in a largevolume action.

erably roediluted to such a degree that the concentration of the slurry will not exceed the concentration of the diluted sludge as described above. IHighly dilute solutions are desirable in order to obtain effective solution of the magnesia values during the subsequent carbonating Thehighly diluted slurry is then subjected to a carbonation actiontin a series of low concentration carbonating tanks. In this operation carbon dioxide gas is passed through the diluted lime sludge mixture and/or other magnesia-containing materials until the water-insoluble magnesium compounds have been converted into water-soluble magnesium compounds.`

From the low concentration carbonators the sludge is pumped to a thickening or settling tank and the water-insoluble calcium carbonate components and other insoluble components are pei-- mitted to settle to the bottom of the tank. 'I'he highly diluted magnesium bicarbonatesolution is separated from the settled calcium carbonate sludge and pumped back into the manufacturing cycle either into, the slaker or into an accumulator thickener tank. If no calcium carbonate components are present, as, for example, when treating vwaste trim or other basic magnesium carbonate materials, the settling tank operation may be omitted, since no calcium carbonate is pres# ent. vIn this case the magnesium bicarbonate solution may be pumped directly to the slaker or into the accumulator. The recovered magnesiumv bicarbonate solution will react with the calcium hydroxide or slaked'calcined dolomite in the ac- 3' ing carbon dioxide through the mixture, and after carbonation of the diluted sludge 98 per cent of the magnesia was found to be in solution.

'I'he magnesium bicarbonate solution was then separated from the undissolved solids and added to the hydrated slaked dolomite entering the process. f

Another example of the 4application of my ini vention, in order to illustrate the -eiectiveness thereof, is directed to the treatment of a mixture of plastic and calcined magnesium oxide. In

cumulator to form a substantially water-insoluble magnesium hydroxide.

The recovered magnesium bicarbonate solution may be reacted, 'if desired, with other watersoluble hydroxides, such as hydroxdes .of the alkaline earth vmetals or the alkali metals. Among the water-soluble hydroxides of the alkaline earth metals, such compounds as barium hydroxide or strontium hydroxide are 'satisfactory compounds to precipitate the magnesium bicarbonate as magnesium hydroxide. Hydroxides of alkali metals include sodium hydroxide, potassium hydroxide and ammonium hydroxide, and may be vused in place of the calcium hydroxide described-in the example. Calcium lwdroXide is ordinarily used in my operation, but it is understood that otherhydroxides as described above maybe used to precipitate the magnesium bicar-v bonate solution and areincluded within the scope of the present invention. Calcium hydroxide may ordinarily be considered sparingly soluble as compared to the solubility of, forexample, sodium hydroxide, but when compared with the substantiallyinsoluble magnesium hydroxide it may be construed as substantially water-soluble,

and for the purpose of the present specification and claims, calcium hydroxidel will be defined as water-soluble in con-Junction .with vother lwaterf soluble hydroxides described above.

Asian illustration ofthe procedure showing the effectiveness and high recovery of magnesia values in lime sludge, 1000 gallons of lime sludge 'was diluted with 6500 gallons of water. The diiuted mixture had a concentration of magnesium carbonates of 0.67 percent expressed as magnesium basic carbonate. The solids in the sludge were vfound by gravimetric analyses to be composed ot 78 percent vcalcium salts and 22 per cent magnesium salts. The diluted mixture was then subjected to a carbonating action by passl this operation a slurry of a mixture of plastic and cal-cined magnesium oxide' in suuicient quantity to give a total concentration of 1.75 per cent, expressed as magnesium basic carbonate (0.65 per cent plastic and 1.10 per cent magnesium oxide), was carbonated with substantially pure carbon dioxide gas at 40 lbs. gauge pressure. As a result o f the carbonating action, 84 per cent of the total available magnesia entered into solution iniorty minutes. cordance with my invention the same quantity of plastic (0.65 per cent available magnesia expressed as magnesium basic carbonate) was subjected to carbonation at 40 lbs. lgauge pressure for twenty minutes, all the plastic was dissolved.

The magnesiumoxide was then added to the solution in the same quantity as used above, that is suiicient to give a potential concentration of 1.10l per cent, or a total concentration of 1.75 per cent. The magnesium oxide suspendedin the magnesium bicarbonate .solution was then ysubjected to further carbonation, and after twenty vminutes of carbonation at 40 lbs. gauge pressure,

ables the use of higher concentrations of magnesium bicarbonate solutions. Even though magnesium compounds tend to precipitate from the solution at the higher concentrations, the magnesia values are readily recovered from the waste waters and sludges. Furthermore, since higher concentrations of bicarbonate solutions can be usedsatis`factorily without loss of too much of the magnesia value, less steam will be required to heat the magnesium bicarbonate solution in the precipitator due to the smaller volume of solution. Thus, when all the factors are considered, itwillfbe evident that a substantial saving in operation and increased capacity are effected.

It is desirable that the recovery of the magnesiumcompounds from the lime sludge material or other magnesia products be effected at very low concentrations. If the solution is diluted to such a degree that a very low concentration of magnesium compounds exists in the solution, substantially complete recovery of the magnesium compounds is obtained. ,Y .The concentration of the carbon dioxide gas used 'in the carbonation of thejwasteinaterials and other sludges is preferablyhigh. It is also desirable to have relatively high pressures forcing the gas through the carbonatorin order to obtain effective carbonation of the magnesium compounds. If the gas used in the resaturation reaction has a carbon .dioxide content in excess of 50 per cent by volume, excellent results will be obtained. If the concentration is lower, the reaction will be slower, but satisfactory results will be obtained although more time will be required before the treated materials may be added to the fresh stock in the accumulator thickener. Gases.

' insoluble magnesium compounds having a magnesium concentration of about 0.20.6% expressed as magnesium oxide, subjecting the aqueous suspension to the action of carbon dioxide gas under conditions adapted to convert the water insoluble magnesium compounds into water soluble magnesium bicarbonate, separating Water insoluble compounds from the magnesium bicarbonate solution, admixing a water soluble hydroxide and magnesium hydroxide with the magnesium bicarbonate solution, reacting the magnesium bicarbonate solution with the water soluble hydroxide to form r substantially water insoluble magnesium hydroxide, and subjecting an aqueous suspension of the magnesium hydroxide having a concentra.- tion calculated as magnesium oxide above 0.6% to the action o f carbon dioxide under conditions adapted to produce magnesium bicarbonate.

2. The process of recovering magnesia in a suitable form for use in manufacturing molded magnesium carbonate products which comprises, forming a dilute aqueous suspension including calcium carbonate, magnesium carbonates and water soluble magnesium bicarbonate, said sspension having a magnesium concentration of substantially 0.2%-0.6% expressed as magnesium oxide, subjecting the aqueous suspension to the action of carbon dioxide gas to convert the water insoluble magnesium carbonates into water soluble magnesium bicarbonate, separating insoluble calcium carbonate from the magnesium bicarbonate solution, adding calcium hydroxide and magnesium hydroxide to the magnesium bicarbonate solution, reacting the magnesium bicarbonate solution with the calcium hydroxide to form substantially water insoluble magnesiumv hydroxide, and subjecting an aqueous suspension of the magnesium hydroxide having a concentration calculated as magnesium oxide above 0.6% to the action of carbon dioxide under conditions adapted to produce magnesium bicarbonate.

nate, heating the magnesium bicarbonate liquor to precipitate magnesium carbonate, concentrating the magnesium carbonate by filtration to a molding consistency andv molding, curing and trimming shaped products to speciiied dimensions, formingfa dilute aqueous suspension of 'magnesia values in the manufacture of magnesium carbonate insulation products, which comprises, slaking calcined dolomite to form a dilute aqueous slurry of magnesium hydroxide and calcium hydroxide, subjecting the slurry to a primary carbonation with carbon dioxide to produce soluble magnesium bicarbonate and insoluble calcium carbonate, separating the calcium carbooxideunder conditions adapted to convert substantially all of the magnesia values to soluble magnesium bicarbonate, separating the dilute magnesium bicarbonate liquor thus formed from any insoluble residue of the treatment, and mixing the dilute bicarbonate liquor, water, and calcined dolomite in the hydrating zone in such proportions as to give a 'magnesium oxide content above .6% in the slurry suspension subjected to the primary carbonation treatment.

5. 'I'he cyclic process of preparing magno-sia in a form suitable for use in manufacturing molded magnesium carbonate products, which comprises calcining a dolomite ore forming a mixture of calcium oxide and magnesium oxide, hydrating said oxides and forming a dilute aqueous suspension of calcium and magnesium hydroxides, subjecting the mixed hydroxides to the carbonating action of carbon dioxide'gas and separating the thus formed water-soluble magnesium bicarbonate solution from the precipitated calcium carbonate sludge, and recovering magnesia values including soluble bicarbonate and insoluble carbonates from the said sludge by forming an aqueous slurry suspension of the sludge with a magnesium concentration calculated as magnesium oxide of less than 0.6%, subjecting the aqueous slurry mixture to the action of carbon dioxide gas to convert theprecipitated magnesium carbonate to a water-soluble magnesium bicarbonate, separating the water-insoluble calcium carbonate from'the magnesium bicarbonate solution, and

finally mixing the magnesium bicarbonate solution, water, and the calcined dolomite in the hydrating zone, in such proportions as to provide a magnesium-oxide content above 0.6% in the suspension subjected to the primary carbonation treatment.

.THOMAS C. ATCHISON. 

